1. Field of the Invention
The present invention is directed to image sensors, such as CIDs, CCDs, and the like. More particularly, it is directed to an image sensor capable of functioning in a plurality of resolution modes.
2. Description of the Related Art
Image scanners convert a visible image on a document or photograph, or an image in a transparent medium, into an electronic form suitable for copying, storing or processing by a computer. An image scanner can be a separate device or an image scanner may be a part of a copier, part of a facsimile machine, or part of a multipurpose or all-in-one device having printing, scanning, copying and or faxing functionality. Reflective image scanners typically have a controlled source of light, and light is reflected off the surface of a document, through an optics system, and onto an array of photosensitive devices. The photosensitive devices convert received light intensity into an electronic signal. Transparency image scanners pass light through a transparent image, for example a photographic positive slide, through an optics system, and then onto an array of photosensitive devices. Common photosensor technologies include Charge Coupled Devices (CCD), Charge Injection Devices (CID), Complementary-Metal-Oxide (CMOS) devices, and solar cells. Typically, for a CID or a CMOS array, each photosensitive element is addressable. In contrast, CCD line arrays commonly serially transfer all the charges, bucket-brigade style, from each line array of photosensitive elements to a small number of sense nodes for conversion of charge into a measurable voltage.
In general, there is an ongoing demand for increased resolution and speed, improved color quality and image quality, and reduced cost, demands that often directly conflict and require trade-offs. In general, image scanners use an optical lens system to focus an image onto an array of photosensors. Photosensor arrays typically have thousands of individual photosensitive elements. Each photosensitive element, in conjunction with the scanner optics system, measures light intensity from an effective area on the document defining a picture element (pixel) on the image being scanned. Optical sampling rate is often expressed as pixels per inch or “ppi” (or pixels per millimeter) as measured on the document (or object, or transparency) being scanned. Optical sampling rate as measured on the document being scanned is also called the input sampling rate. Photosensor assemblies for linear (as opposed to two-dimension) image scanners commonly have three or four line arrays of sensors, with each line array receiving a different band of wavelengths of light, for example, red, green and blue. Each line array may be filtered, or white light may be separated into different bands of wavelengths by a beam splitter. Typically, the pitch (spacing of individual photosensor elements) is the same for each line array, and typically the pitch is set to provide a specified native input sampling rate. The native input sampling rate is determined by the optics and the pitch of the individual sensors. A scanner operator can select a sampling rate that is less than the native input sampling rate by simply dropping selected pixels, or by using digital resampling techniques. Alternatively, a scanner operator can select a sampling rate that is greater than the native input sampling rate, where intermediate values are computed by interpolation. Typically, all the charges or voltages are read from the photosensor array, and are then digitized, and then subsampling or interpolation is performed on the resulting digital pixel data.
Smaller sensor areas can provide higher input sampling rates, but other measures of image quality, and in particular color quality, as measured by signal-to-noise, may be reduced. If an input sampling rate is selected that is lower than the native input sampling rate, then the signal-to-noise may be improved by averaging samples. Analog signals from adjacent sensor areas can be added, or digital values can be averaged after analog-to-digital conversion. Adding M samples improves the signal-to-noise ratio by the square root of M. Typically, adding analog signals requires the signal levels to be relatively small before adding to avoid saturating a charge element, so that analog averaging is typically used for speed (fewer conversions) rather than for improvement in signal-to-noise ratio. Scanning speed is affected by multiple factors: exposure time, shift time of registers multiplied by number of pixels being shifted, output amplifier speed, and analog-to-digital conversion time.
As imager sensors continue to increase in resolution, the actual imaging elements become smaller. This smaller element has a lower sensitivity to light and requires a larger exposure time to maintain a good signal to noise ratio (SNR), and this impacts image quality.
Current technology exists in which an image sensor contains both high resolution and low resolution imaging elements built on a single piece of silicon substrate. The low resolution imaging elements are much larger than the high resolution imaging elements and therefore have much more sensitivity to light. This allows the imager to operate at higher speeds while still maintaining good image quality. This technology is generally referred to in the industry as a “dual mode sensor”. U.S. Patent Publication No. 2002/0093694 discloses a photosensor assembly comprising a first array of photosensor elements, each photosensor element in the first array having a first size; a second array of photosensor elements, each photosensor element in the second array having a second size, wherein the first size and second size are substantially different.
The prior art also includes using multiple imaging elements that are the same size which are then summed together, on a single piece of silicon, to create a larger “superpixel”. U.S. Pat. No. 6,687,026 discloses such a design.
U.S. Patent Publication No. 2004/0109075 discloses a number of prior art arrangements calling for two or more rows of imaging elements of the same size, at least one row being staggered relative to another row. As is known to those skilled in the art, by appropriate spatial staggering and appropriate timing for receiving and summing signal charges, one can achieve a pixel resolution greater than that of the native resolution of the imaging elements.
FIG. 1 shows a portion of a prior art image sensor 100 in which two rows of imaging elements are staggered. It is understood that the image sensor 100 preferably is formed on single piece of silicon. It is further understood that FIG. 1 depicts the layout for one color, it being understood that this layout is repeated for each color in a color copier, scanner or all-in-one unit.
The image sensor 100 of FIG. 1 comprises two rows 102, 104, each row having a plurality of linearly-arranged imaging elements. Imaging element row 102 is shown to have individual imaging elements 122A, 122B, 122C, 122D, etc., while imaging element row 104 is shown to have individual imaging elements 124A, 124B, 124C, 124D, etc. Shaded regions between adjacent imaging elements in each row depict a physical spacing between the imaging elements. Most notably, in the prior art image sensor 100, all the imaging elements in the various rows 102, 104 are of equal size. The imaging elements in any one row are configured and dimensioned to provide some native resolution, such as 300 ppi, 600 ppi, or some other resolution. Without loss of generality, one can consider the individual imaging elements in the prior art image sensor 100 to have a native resolution of 600 ppi.
The image sensor 100 also comprises two shift registers 112, 114 whose final outputs are directed to output amplifiers 172, 174, respectively. Each shift register 112, 114 comprises a plurality of linearly-arranged shift register elements, respectively designated 132A, 132B, 132C, 132D, and 134A, 134B, 134C and 134D in the figure. It is understood that there typically are many more than just four such shift register elements; designs incorporating hundreds, if not thousands, of such shift register elements (and imaging elements) is not atypical. As also seen in FIG. 1, the imaging element rows 102, 104 pass their charge outputs to the shift registers 112, 114 via transfer gate circuitry 162, 164, respectively.
Furthermore, as seen in the prior art orientation of FIG. 1, the imaging element rows 102 and 104 and their corresponding shift registers 112, 114 are arranged parallel to one another. Shift registers 112 and 114 are at opposite extremes of the layout. Beginning at shift register 112 and traversing the sensor are, in order, imaging element row 102, imaging element row 104 and shift register 114.
As depicted by arrows 152 the charge from each imaging element of row 102 is provided to a shift register element in row 112, while as depicted by arrows 154 the charge from each imaging element of row 104 is provided to a shift register element in row 114. More particularly, each imaging element 122A, 122B, 122C, 122D in row 102 outputs sensor charge via a one-to-one mapping to a corresponding shift register element 132A, 132B, 132C, 132D, respectively. Similarly, each imaging element 124A, 124B, 124C, 124D in row 104 outputs sensor charge via a one-to-one mapping to a corresponding shift register element 134A, 134B, 134C, 134D, respectively. All the shift register elements in the shift registers 112, 114, respectively, are of the same, first design. In a preferred embodiment, row 104 and its corresponding shift register 114 serve as a native resolution CCD imager 160 having 600 ppi resolution.
A clock 196 provides the prior art image sensor 100 with a clock signal 196A that is presented to both shift registers 112, 114 to regulate the reading of charges and shifting of the charge values. A controller 198 provides control signals to the clock, transfer gates, output amplifiers and other components.
Imaging element rows 102 and 104 are staggered relative to one another. More particularly, row 102 is laterally shifted by one-half pixel width relative to imaging element row 104. This means that the output of their corresponding shift registers can be used for interpolation, thereby providing twice the spatial resolution of either row 102 or 104 by itself. Thus, if imaging element rows individually provide a native resolution of 600 ppi, the staggered high-resolution CCD imager 170 formed by these two rows provides a resolution of 1200 ppi.